The present invention is directed to a known cardiac pacing function, the so called "fallback" mode, and the resynchronisation of the cardiac ventricular stimulation to the atrial rhythm when the atrial rate returns to normal at the end of a phase of fallback.
The fallback mode and resynchronization have particularly been discussed in FR-A-2,544,989 as well as EP-A-0 488840 and its corresponding U.S. Pat. No. 5,226,415, all commonly assigned to ELA Medical, the assignee of this invention. These documents describe a mode of processing atrial extra systoles ("AES") and functioning in fallback, corresponding to what is implemented in the commercial pacemaker product that is sold under the mark CHORUS II 6234 by ELA Medical, Montrouge France.
The myocardium can be subjected to what is referred to as a trouble of the atrial rhythm ("ToAR"), a generic term that covers various atrial arrhythmias including, but not limited to, tachycardia, fibrillation, and flutter. TOAR is characterised, at its detection, by a rapid atrial rhythm.
In the absence of ToAR, the pacemaker normally operates in a DDD pacing mode, that is to say with the atrium and ventricle associated (i.e., synchronized).
When one detects a ToAR, that is to say, essentially, when the atrial rhythm exceeds an acceptable (or threshold) level, the pacemaker switches to a mode called "de-synchronisation" (or "atrial-ventricular dissociation"). In the desynchronisation mode, the pacemaker stimulates the ventricle independently of the detected atrial rhythm, because the excessive atrial rhythm is considered to be pathological.
When the atrial rhythm returns to the acceptable level, the pacemaker operates a "re-synchronisation" (also called an "atrial-ventricular re-association") to return, in a progressive manner, to function in the DDD mode. It is necessary that the re-synchronisation be progressive to detect possibly an association in a 2:1 (Wenckebach) mode, in which case it would not be necessary to proceed to the re-association.
The invention concerns more particularly the control of the re-synchronisation in the fallback mode.
In known devices, such that those described in aforementioned patent documents, one re-synchronises when one no longer detects a ToAR arrhythmia. But it happens frequently that an atrial fibrillation ("AF"), originating with a ToAR, degenerates over time, and, although it remains present, in fact it no longer can be detected by the pacemaker. In this case, the pacemaker, after being correctly changed to a fallback mode (de-synchronisation) at the beginning of the ToAR arrhythmia, wrongly interprets the absence of detection of the ToAR as a disappearance of the ToAR arrhythmia, and then re-synchronises to operate in a DDD mode.
In others words, particular signal characteristics of an AF, namely, a lower amplitude, a very irregular rhythm, etc., can lead to a loss of detection of the signal that is interpreted and incorrectly managed by the pacemaker as a disappearance of the corresponding ToAR.